Method and flux for growing single crystals of garnet or ortho ferrites

ABSTRACT

A FLUX CONSISTING OF BAO, B2O3 AND A BARIUM HALIDE SUCH AS BAF2, BAC12, BABR2 OR BAI2 IS USED FOR THE SOLUTION GROWTH OF SYNTHETIC GARNETS AND ORTHOFERRITES WHEN APPROPRIATE AMOUNTS OF THE PROPER NUTRIENTS ARE DISSOLVED IN THIS FLUX AND ALLOWED TO PRECIPITATE OUT AS SINGLE CRYSTALS. THIS OCCURS EITHER BY SLOWLY COOLING AN INITIALLY UNSATURATED SOLUTION, OR BY CONTINUOUSLY DISSOLVING NUTRIENT IN A HOTTER REGION OF THE SOLUTION, TRANSPORTING IT DOWN A TEMPERATURE GRADIENT AND ALLOWING IT TO PRECIPITATE OUT IN THE COOLER REGION OF THE SOLUTION. THE PRECIPITATION MAY OCCUR ON A SEED CRYSTAL OF THE SAME MATERIAL PLACED IN THE COOL REGION OF THE SOLUTION (A BULK GROWTH PROCESS), OR IT MAY OCCUR HOMOEPITAXIALLY ON A THIN SUBSTRATE OF THE SAME MATERIAL OR HETEROEPITAXIALLY ON A SUBSTRATE OF DIFFERENT COMPOSITION.

Oct. 10, 1972 R. HISKES 4 3,697,329

METHOD AND FLUX FOR GROWING SINGLE CRYSTALS OF GARNET OR ORTHOFERRITES Filed Jan. 11, 1971 MOLE PERCENT mo;

igure 2 INVENTOR RONALD HISKES United States Patent U.S. Cl. 117-234 9 Claims ABSTRACT OF THE DISCLOSURE A flux consisting of BaO, B 0 and a barium halide such as BaF BaCl BaBr or Bal is used for the solution growth of synthetic garnets and orthoferrites when appropriate amounts of the proper nutrients are dissolved in this flux and allowed to precipitate out as single crystals. This occurs either by slowly cooling an initially unsaturated solution, or by continuously dissolving nutrient in a hotter region of the solution, transporting it down a temperature gradient and allowing it to precipitate out in the cooler region of the solution. The precipitation may occur on a seed crystal of the same material placed in the cool region of the solution (a bulk growth process), or it may occur homoepitaxially on a thin substrate of the same material or heteroepitaxially on a substrate of different composition.

BACKGROUND AND SUMMARY OF THE INVENTION An number of fluxes have been used in the past to grow single crystals of synthetic garnets and orthoferrites. Usually these fluxes were unary, binary or ternary systems containing either lead oxide or barium oxide. The fluxes using lead oxide have two major drawbacks: (1) any free lead in the system attacks the platinum crucible; (2) the flux is quite volatile in the temperature region best suited for growing the aforementioned crystals. The binary barium oxide systems also have the disadvantages that, except for a narrow composition range near the eutectic point: (1) the flux melting point is so high that a very limited temperature range is available for crystal growth; and (2) the flux viscosity is high, hence mass transport of nutrients is impeded, thus impeding crystal growth.

The present invention uses a ternary flux containing barium oxide, boron oxide and a barium halide compound such as barium fluoride. The addition of barium fluoride to the combination of barium oxide and boron I oxide produces a flux with several distinct advantages: (1) the melting point is decreased, resulting in a greater range of temperatures for crystal growth; (2) the viscosity is decreased, increasing the rate of crystal growth; (3) the flux is less volatile and more chemically stable than fluxes containing lead oxide; (4) the flux does not attack the platinum crucible; (5) the solubility of nutrients is increased.

There are several suitable methods for growing various synthetic garnets and orthoferrites in single crystal form. Such methods include Bridgman growth from a melt, the Czochralski method of pulling from a melt, floating zone techniques, the traveling solvent method, vapor phase epitaxial growth, and growth from solution. When growing from solution the crystals may be precipitated out of a slowly cooled solution comprised of the fiux and the constituents of the crystal material. Alternatively, a crystal may be grown by liquid phase epitaxy, either homoepitaxially on a seed crystal of the same material or heteroepitaxially on a substrate of different composition from the crystal being grown, e.g., the growth of YFeO on YA1O or of Y Fe O on Gd Ga O or the growth of heavier rare earth garments on neodymium galluim garnet (Nd Ga O The term seed crystal is used generically in this application to refer to any single crystal material on which additional crystalline material is grown. A seed crystal may be of irregular shape or it may be relatively thin, flat sheet, as connoted by the word substrate; and a seed crystal may be of the same or different material from that grown on it. In either homoepitaxy or heteroepitaxy, the substrate is placed in a relatively cool portion of the melt, which is saturated with the crystal constituents, while more constituents are constantly added to a warmer portion of the melt. Use of a large area substrate of appropriate crystallographic orientation permits large area single crystals to be readily grown. The crystals thus grown may be the garnets, which can be represented by the formula R Me O or R Me (MeO where O is oxygen, Me is a trivalent metal and R is yttrium or one of the rare earth elements of atomic number between 57 and 71 or a mixture of these rare earth elements with each other or with yttrium. Me may be at least one of the elements trivalent iron, gallium, scandium, chromium, cobalt, or aluminum. The orthoferrites may also be grown with the method described herein, the latter being represented by the formula XMeO where O is oxygen and X may be yttrium or one of the rare earth elements of atomic number between 57 and 71.

As is well known in the art, single crystals of ferromagnetic material show enhancement of certain magnetic properties relative to the polycrystalline material. In particular, the resonance lines of single crystal materials are much narrower than those found in the polycrystalline material, this property forming the basis for various types of commonly used microwave devices, for example those devices known as YIG resonators and YIG oscillators. Such single crystal materials are also required for devices of the magnetic bubble type, as described for example in an article entitled Application of Orthoferrites to Domain Wall Devices by Andrew H. Bobeck, Robert F. Fischer, Anthony J. Perneski, J. P. Remeika and L. G. Van Uitert in IEEE Transactions on Magnetics, vol. MAG-5, No. 3, September 1969, pp. 544553.

Maetrials requirements for the magnetic bubble memories are very stringent, since such devices depend on the ease and reliability with which cylindrical magnetic domains can be generated and moved through thin platelets of single crystal material. The mobility of such domains depends very strongly upon the nature and number of defects in the crystal, such as inclusions, voids, dislocations, twin planes, chemical homogeneity, etc. Promising materials for this application include mixed garnets such as combinations of rare earth elements replacing Y in Y Fe O while some of the Fe is replaced by Ga or Al to produce, for example,

which is required in layers only a few microns thick of specific crystallographic orientation.

Crystals can be grown from a solution consisting of a flux and nutrients by a number of methods, which include growth of bulk crystals and the epitaxial growth of thin platelets. For example, a solution may be heated to the point where it is undersaturated with respect to the crystal constituents and then slowly cooled at a rate of /2 to 2 C. per hour to permit random nucleation and growth of single crystals. Alternatively, the solution may be kept in a fixed temperature gradient with constant average temperature and seeded either with a seed crystal of the same composition (homoepitaxy) or a seed crystal of dilferent composition (heteroepitaxy). The size, shape and crystallographic orientation of the seed crystal may of course be tailored for the particular desired characteristics of the grown crystal. A third alternative is bulk growth in a fixed temperature gradient, where a seed may or may not be present, and the crystals either nucleate randomly in the cooler portion vof the solution or begin growing upon the seed. The crystals thus nucleated are allowed to grow freely to any desired size, properly nourished by a constant supply of nutrient in the hotter region of the solution. Another variation of the solution growth method is quite similar to the standard zone refining technique used for producing single crystal semiconductor materials. A traveling zone of molten flux is made to pass through a polycrystalline charge of the crystal material. The single crystal grows on the trailing edge of the zone. Alternatively, apolycrystal may be moved through a molten zone of flux to form a single crystal. The requirement for very thin, large area, defect-free crystals makes heteroepitaxial growth the best choice for growth of these materials, and epitaxial growth can only be well controlled when the flux is stable, i.e.,. nonvolatile, nonreactive and does not change in composition over a period of time suflicient for the growth process.

As explained in more detail below, a flux consisting of Ba'O-B O BaZ where Z may be any one of fluorine, chlorine, bromine or iodine, exhibits such stability.

DESCRIPTION OF THE DRAWINGS FIG. 1 shows a ternary phase diagram for the crystal growing flux; and

FIG. 2 shows a solubility curve for YFeO in a BaO-BaF --B O tlux.

DESCRIPTION OF THE PREFERRED. EMBODIMENT FIG. 1 is a ternary phase diagram for B aO-B O -B 322 Lines 10, 12, 14 and 16 delineate an area 11 defining the range of compoistions suitable for growing single crystal synthetic garnets and orthoferrites. The area to the right of line 12 is not suitable since the viscosity of the melt is too great to allow sufficient mass transport of nutrients for a reasonable rate of crystal growth. In addition, undesirable borate formation takes place to the right of line 12, and the desired compounds are believed to be unstable in that region. The area to the left of line 10 is not suitable since the liquidus temperature is too high to allow a reasonable temperature range over which to cool the melt for precipitating out large single crystals. Also, it is believed that iron containing crystals begin to lose oxygen at that liquidus temperature, which, for example, is 1500 C. at point 18 (81.5 percent BaO, 18.0 percent B 0.5 percent BaZ In addition, the .large proportion of BaO and BaF present in the flux to the left of line will corrode a platinum crucible. Point 20 is a BaOB O' eutectic and points 22 and 24 are B O BaF eutectics. Line 26 is an experimentally determined trough in the liquidus surface. It is believed that there are other troughs like line 26 in area 11, and there may also be a ternary eutectic. Crystals of YFeO for example, may be grown from a flux of composition 28 in FIG. 1 when appropriate equimolar amounts of the nutrients Y O and Fe 0 are added to the flux. The proper amounts of the nutrients are dictated by the temperature of crystal growth and a curve of solubility vs. temperature asshown, for example, in FIG. 2.

The amounts of nutrient constituents dissolved in the flux for the growth of garnets or orthoferrites need not be present in stoichiometric ratios as outlined in the example above, but rather may be any composition which lies within the limits of the phase field or composition region of stability for the compound in question. These limits are defined as that region in the phase diagram within which the solution plus nutrient, when slowly cooled, will precipitate out the desired compound. These stability regions must be experimentally determined since they vary for each compound and are a function of flux composition.

It is believed that other barium halide compounds, such as Bacl BaBr BaI will also lower the liquids temperature without affecting the stability of the flux. Fluxes using a combination of barium halide compounds, for example BaF and BaBr as well as mixed halide compounds such as BaClF, are believed similarly suitable. In addition, the BaF increases the solubility of the nutrients in the flux at a given temperature and it changes. the ionic character of the flux-nutrient system. Increased solubility of nutrients means that crystal growth can proceed at a greater rate, since more nutrients are present in the melt; and also increases the yield of single crystal material.

Both BaO and BaF are ionic crystals, however, BaF is considerably more ionic. It is believed that this greater ionicity contributes to greater solubility of Fe" in the flux. There is some evidence of the presence of Fe in the lattice of YFeO This substitution for Fe is undesirable since it introduces inhomogeneities in the crystal structure which make the magnetic properties of the crystal hard to control. However, F- inhibits the formation of Fe because the F- will replace some of the 0 which from charge considerations then makes it unlikely that Fe+ will be present. As little as 0.5 mole percent BaF; will supply sufiicient F to effect the inhibition of Fe, and will thereby result in more than a factor of two increase in the size of the crystals grown.

The balance between B 0 and BaF is the major factor in determining the viscosity of the melt. Too large a viscosity, as mentioned before, inhibits mass transport. However, too small a viscosity permits spontaneous nucleation which allows many small crystals to form instead of a few large ones. The appropriate balance can be maintained by limiting the composition of the flux to area 11 in FIG. 1. Similar results to those described above can be achieved by substituting V 0 or M00 for B 0 and SrO or Bi203 fOI' BaO.

As set forth in the summary, there are many compounds which can be grown using the flux herein described. A few specific examples will serve to illustrate the use of the method for growing single crystal synthetic garnets and orthoferrites.

EXAMPLE 1 A mixture of 24.5 grams of BaO, 11.7 grams of B 0 12.91 grams of BaF 8.9 grams of Fe O and 12.67 grams of Y 0 was heated in a platinum crucible to a temperature of 1285 C. The mixture was then cooled to a temperature of 1050 C. at a rate of 0.85 C. per hour, and the liquid was poured off. The resultant YFeO orthoferrite crystals were cooled in the furnace to 500 C. and then air cooled to room temperature. Any flux residue on the crystals was washed oif with nitric acid at 75 C. The crystals thus produced were approximately 3 by 3 by 5 millimeters in size.

EXAMPLE 2 A mixture of .7498 gram of Eu 0 19.9109 grams of Gd O 5.11 grams of Tb O 4.828 grams of Gd 0 34.39 grams of Fe O 116 grams of BaO', 46.77 grams of B 0 and 51.70 grams of BaF was heated in a platinum crucible to a temperature of 1285 C. The mixture was then cooled to a temperature of 800 C. at a rate of 0.85 C. per hour and the liquid was poured otf. The resultant Eu gGd Tb Ga FemO garnet crystals were cooled in the furnace to room temperature, and any flux residue on the crystals was washed 01f with 20% nitric acid at 75 C.

EXAMPLE 3 A mixture of 118.7 grams of B 0 101.6 grams of BaF 469.5 grams of BaCO 103.3 grams of Y O and 73.1 grams of F6 0 were heated in a ten-centimeter deep by 7.6 centimeter diameter platinum crucible supported in a furnace having a vertical temperature gradient of 7.6" C. per centimeter, the lowest temperature, 1115 C., being at the top of the crucible. A by 5 by 5 millimeter YFeO seed crystal was suspended in the top portion of the liquid in the crucible and rotated at 50 r.p.m. There were 14 /2 sintered pellets of YFeO each weighing 0.75 gram, lying at the bottom of the crucible to supply additional YFe0 as the crystal grew. &After seven days the liquid was poured oil and the YFeO orthoferrite crystal was cooled to room temperature in the furnace and the flux residue was washed oil? with 20% nitric acid at 75 C. The new growth of YFeO on the seed crystal was 8 by 6 by 6 millimeters.

I claim:

1. A flux for growing single crystals of a material selected from the group consisting of synthetic garnets and orthoferrites, the flux comprising barium oxide, boron oxide and at least one of the barium halide compounds represented by the formula BaZ where Z is at least one of the elements fluorine, chlorine, bromine and iodine, the composition of said flux being defined within an area on a ternary phase diagram having as its coordinates mole percent BaO, mole percent B 0 and mole percent BaZ said area being defined by a quadrilateral having at its corners the four points defined by (1) 81.5 percent BaO, 18.0 percent B 0 0.5 percent (2) 50.0 percent BaO, 49.5 percent B O 0.5 percent BaZ (3) 0 percent BaO, 75 percent B 0 25 percent BaZ (4) 0 percent BaO, percent B 0 90 percent BaZ 2. A method for epitaxially growing single crystals of a material selected from the group consisting of synthetic garnets and orthoferrites which comprises heating the constituent components of said material in a flux as described in claim 1 to form a melt and situating a seed crystal within said melt, whereby said material is deposited on said seed crystal in crystalline form.

3. The method of claim 2 in which said synthetic garnet material is a compound represented by the formula R Me O where O is oxygen, Me is at least one trivalent metal selected from the group consisting of trivalent iron, gallium, scandium, chromium, cobalt and aluminum, and R is at least one member selected from the group consisting of yttrium and rare earth elements having an atomic number within the range 57 to 71, and said seed crystal is a garnet represented by said formula R Me O' 4. The method of claim 2 in which said synthetic orthoferrite material is a compound represented by the formula XMeO Where O is oxygen, Me is at least one trivalent metal selected from the group consisting of trivalent iron, gallium, scandium, chromium, cobalt and aluminum, and X is at least one member selected from the group consisting of yttrium and rare earth elements having an atomic number within the range 57 to 71, and said seed crystal is an orthoferrite represented by said formula XMeO 5. The method of claim 3 wherein Me is trivalent iron, R is yttrium, Z is fluorine, and said seed crystal is 3 5 l2- 6. The method of claim 4 wherein Me is trivalent iron, X is yttrium, Z is fluorine, and said seed crystal is YFeO 7. The method of claim 3 wherein Me is trivalent iron, R is yttrium, Z is fluorine, and said seed crystal is 611 635012.

8. The method of claim 4 wherein Me is trivalent iron, X is yttrium, Z is fluorine, and said seed crystal is YA1O 9. The method as in claim 3 wherein said seed crystal is Nd Ga O References Cited UNITED STATES PATENTS 3,079,240 2/1963 Remeika 23305 3,115,469 12/ 1963 Hamilton 23-51 3,117,934 1/1964 Linares 23-300 3,386,799 6/1968 Grodkiewicz et a1 235l 3,429,818 2/1969 Benedetto et al. 252-62.57 3,486,937 12/1969 Linares 117235 3,630,667 12/1971 Shirk 2351 NORMAN YUDKOFF, Primary Examiner R. T. FOSTER, Assistant Examiner US. Cl. X.R.

117-235, 169 R; 25262.57; 23-51 R, 300, 301 SP, 305 

